Many devices connected to Control Center can be controlled directly and reliably using IP (internet protocol), BT (Bluetooth) or HDMI-CEC (Consumer Electronics Control) protocol. However, there are also many devices that don't use these methods (e.g. TV, cable box, TiVo, many BluRay players, some AVRs, etc) so Control Center resorts to using IR (infrared) control. IR signals are digitally-coded pulses of infrared radiation to control functions such as power, volume, and channel changes. These devices require a direct "line of sight" with an IR transmitter to work properly. If the "line of sight" is changed/blocked, or the transmitter is moved out of alignment, IR control can become sporadic or stop working altogether.
Due to the nature of IR, it can be difficult to troubleshoot, because it cannot be seen with the naked eye. Try using a digital camera (your phone's camera should work; iPhones use the front camera) to "see" the Infrared signal blasting from the Control Center box as well as any attached emitters. The Control Center unit has emitters in each corner that will light up when issuing IR commands like Volume UP/Down to other devices. This is also a good method to determine if your IR transmitter is actually sending a signal or not.
Please go here for various IR troubleshooting steps.